P.H. Key

Dynamics of excimer laser ablation of superconductors in an oxygen environment

The plume dynamics for excimer laser ablation of Y‐Ba‐Cu‐O in an O2 atmosphere have been studied using streak photography and spectroscopy. At pressures ≳1 mbar the expansion resembles a blast wave driven by the ablation products with mixing and reaction at the contact surface. A simple model for the plume range is developed which shows agreement with experiments.

An investigation of laser ablation and deposition of Y-Ba-Cu-O in an oxygen environment

Abstract Streak photography, visible-UV emission spectroscopy, infrared emission and time-integrated photography have been used to study the laser ablation of Y-Ba-Cu-O in an oxygen environment. At O 2 pressures ≥ 1 mbar the dynamics of the plume become relatively complex, with the expansion resembling a blast-wave with strong mixing and reaction at the contact surface. The range of the ablated material has been measured and compared with an adiabatic expansion model which gives a reasonable description of the results. Preliminary experiments suggest that the quality of deposited films is critically dependent upon the relative position of substrates with respect to the plume range.

Fast time-response photoacoustic studies and modelling of KrF laser ablated YBa2Cu3O7

Abstract The transient pressure produced at the surface of YBa2Cu3O7 samples ablated using a KrF laser has been measured with nanosecond time resolution. It is observed that ablation commences within the ∼ 10 ns laser pulse and leads to large amplitude stress waves propagating in the sample. The results are analysed using a vaporization model and it is tentatively concluded that relatively high average mass species are produced in the initial ablation step. A scheme for explosive removal to account for this behaviour is put forward.

Investigation of excimer laser ablation of ceramic and thin film Y‐Ba‐Cu‐O using nanosecond photoacoustic techniques

Nanosecond photoacoustic measurements of excimer laser ablated ceramic and thin film Y‐Ba‐Cu‐O superconductors are reported that provide information on the ablation timescale threshold and surface pressure. Results for the surface pressure are found to be inconsistent with a simple surface vaporization model, suggesting that a different removal mechanism is operative. The outline of an alternative mechanism based on the concept of explosive removal is put forward.

A comparative study of the effects of laser wavelength on laser removal of chlorinated rubber

This paper compares the use of different lasers for the removal of Chlorinated Rubber (CR) coatings from concrete surfaces. The work involves the investigation of basic phenomena and reactions of CR to various laser wavelengths, including CO2 (10.6 μm), diode (810 nm) and ArF excimer (193 nm) laser radiation. Characteristics of beam absorption, residues and the thermal effects on the materials are analysed by optical microscopy, SEM and EDX. By calculating the thermal loading (γ) of the materials for the different lasers it has been revealed that the diode and excimer laser removal processes have very similar thermal loading with a γ value of 1.2 kJ/cm3 for the excimer laser and 1.5 kJ/cm3 for the diode laser. The similarity of the two arises from the fact that this thermal loading is of the same order of magnitude as the dissociation energy of the primary bonds. The thermal loading for the CO2 laser process is around 100 J/cm3, an order of magnitude smaller than the other two systems

Observation of magnetic-field-enhanced excitation and ionization in the plume of KrF-laser-ablated magnesium

The influence of a magnetic field on the plume produced by KrF-laser ablation of magnesium in vacuum has been investigated using time-integrated photography, streak photography, spectroscopy and charge probes. Line emission spectra in the ≈200–600 nm interval and effective stream velocities for the plume obtained from the spatiotemporal emission from specific neutral and ion lines are reported. Time of flight velocities are also deduced from measurements using simple charge-collector probes. Changes in the plume structure and dynamics, and enhanced emission and ionization are observed in the presence of the field. A qualitative explanation of the results is given in terms of a magnetohydrodynamic model.

A CW CO2 laser substrate heater for superconducting thin-film deposition

Single-crystal MgO and SrTiO3 substrates have been raised to temperatures suitable for deposition and in situ annealing of high-Tc superconducting thin films by the absorption of 10.6 mu m laser radiation. Steady-state temperature control in vacuum and low-pressure oxygen was achieved by varying the mark-space ratio of the RF-excited CO2 laser heater source. This approach allows high temperatures (>or=950 K) to be achieved with relatively low power input (<or=30 W) and maintains deposition chamber cleanliness.

Blast-wave studies of excimer laser ablation of ZnS

Abstract Dye-laser imaging has been used to study the blast-wave created during KrF laser ablation of ZnS in a gaseous environment allowing the temporal and spatial development of the shock wave and contact surface to be visualised. The data suggest that, over short time-scales, a transverse pressure gradient exists across the ablation site and this may contribute to the observed spatially non-uniform ablation rates.

Laser ablation studies of magnesium oxide

Abstract The ablation of single crystal magnesium oxide using a UV-excimer KrF laser at 248 nm has been studied using a variety of diagnostic techniques. Ablation depth rate measurements indicate a threshold fluence for significant material removal of ∼3 J cm -2 although surface damage in the form of micro-cracks is observed below this value. The effective absorption coefficient at 248 nm of 4×10 4 cm -1 , found from the ablation rate, was much greater than observed from small signal transmission measurements. Scanning electron microscopy reveals evidence of thermal removal in the ablation regime probably mediated by a breakdown plasma, and a degree of exfoliation below the ablation threshold. The velocity of selected ablation species has been determined from time-resolved emission spectroscopy and a charge collector probe used to measure the angular distribution of ions in the ablation plume.

Excimer laser ablation and film deposition of Ti:sapphire

ArF laser ablation of Ti:sapphire (Ti:Al2O3) has been investigated as a potential means for micromachining this laser crystal and depositing thin layers for use as active waveguides. Plume spectra recorded for ablation in low pressure oxygen and a preliminary assessment of layers grown by this method are reported.